Electrolyte solution for lithium secondary battery and lithium secondary battery including same

ABSTRACT

An additive for an electrolyte solution improves the electrochemical properties of a lithium secondary battery.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Korean Patent Application No.10-2021-0191208, filed Dec. 29, 2021, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND 1. Field

The present disclosure relates to an electrolyte solution constituting alithium secondary battery and to a lithium secondary battery includingthe same. Specifically, the present disclosure relates to an additivefor an electrolyte solution to improve the electrochemical properties ofa lithium secondary battery.

2. Description of the Related Art

Batteries are energy storage sources capable of converting chemicalenergy into electrical energy or electrical energy into chemical energy.Batteries can be divided into non-reusable primary batteries andreusable secondary batteries. Compared to primary batteries which areused once and discarded, secondary batteries are eco-friendly comparedbecause they can be reused.

Recently, as environmental problems have emerged, the demand for hybridelectric vehicle (HEV) and electric vehicle (EV) with little or no airpollution is increasing. In particular, EVs are vehicles in which theinternal combustion engine has been completely removed, suggesting thefuture direction the world should take.

A lithium secondary battery is used as an energy source for EVs. Alithium secondary battery is largely composed of a cathode, an anode, anelectrolyte, and a separator. In the cathode and anode, intercalationand deintercalation of lithium ions are repeated to generate energy, anelectrolyte becomes a path for lithium ions to move, and in theseparator, the cathode and anode meet to prevent a short circuit in abattery.

In particular, the cathode is closely related to the capacity of thebattery, and the anode is closely related to the performance of thebattery such as high-speed charging and discharging.

The electrolyte is composed of a solvent, an additive, and a lithiumsalt. The solvent becomes a transport channel that helps lithium ionsmove back and forth between the cathode and the cathode. In order for abattery to have good performance, lithium ions must be rapidlytransferred between the cathode and the anode. Therefore, selecting anoptimal electrolyte is very important in order to obtain excellentbattery performance.

In particular, a thin film called solid electrolyte interphase (SEI) isformed on the anode in the chemical conversion process performed duringthe production process of the battery. SEI is a membrane that can passlithium ions but not electrons and prevents battery performance fromdegrading because electrons pass through SEI and induce additionalreactions. In addition, the SEI suppresses the direct reaction of theelectrolyte and the anode and suppresses the separation of the anode.

The additive of the electrolyte is a substance added in a trace amountof 0.1 to 10% with respect to the weight of the electrolyte. Despite thetrace amount added, the performance and stability of the battery aregreatly affected by the additives. In particular, the additive inducesthe formation of SEI on the surface of the anode and plays a role incontrolling the thickness of the SEI. In addition, the additive canprevent the battery from being overcharged and can increase theconductivity of lithium ions in the electrolyte.

On the other hand, the energy density of lithium secondary batterieslargely depends on the characteristics of cathode and anode materials,and it is necessary to develop a suitable electrolyte for the developedcathode and anode materials to exhibit excellent electrochemicalperformance.

Recently, in NCM-based oxide, which is a high-capacity cathode activematerial, the cathode capacity may be increased by increasing the Nicontent or the high voltage of the charging voltage, but the residuallithium (Li₂CO₃ and LiOH) components on the surface of the cathodeaccelerate electrolyte decomposition and also increase a degradationrate due to an increase in interface reactivity with the electrolyte,thereby degrading a lithium secondary battery and rapidly degradingelectrochemical performance.

Therefore, it is necessary to introduce an additive capable of formingan electrochemically and chemically stable SEI.

The matters described as the background art above are only for improvingthe understanding of the background of the present disclosure and shouldnot be accepted as acknowledging that they correspond to the related artalready known to those of ordinary skilled in the art.

SUMMARY

The present disclosure has been proposed to solve these problems, and anobjective of the present disclosure is to provide an electrolytesolution additive capable of improving the electrochemical properties ofa lithium secondary battery by being added to the electrolyte solutionof a lithium secondary battery.

The present disclosure relates to an electrolyte for a lithium secondarybattery containing an electrolyte salt and an organic solvent, theelectrolyte contains vinylene carbonate (VC) represented by thefollowing Formula 1, and the electrolyte further includes an additive,4-(allyloxy)phenyl fluoro sulfate, represented by the following Formula2.

4-(allyloxy)phenyl fluoro sulfate may be included in an amount of 0.1 to1.0% by weight with respect to the total weight of the electrolyte.

VC may be included in an amount of 0.1 to 10% by weight with respect tothe total weight of the electrolyte.

Electrolyte salts may be mixed with any one or two or more compoundsselected from the group consisting of LiPF₆, LiBF₄, LiClO₄, LiCl, LiBr,LiI, LiB₁₀Cl₁₀, LiCF₃SO₃, LiCF_(3.0)CO₂, Li(CF₃SO₂)_(3.0)C, LiAsF₆,LiSbF₆, LiAlCl₄, LiCH₃SO₃, LiCF₃SO₃, LiN(SO_(2.0)C₂F₅)₂, Li(CF₃SO₂)₂N,LiC₄F₉SO₃, LiB(C₆H₅)₄, and Li(SO₂F)₂N(LiFSI).

The electrolyte salt may be included in a concentration of 0.5 M to 1.0M.

The organic solvent may be any one or two or more of solvents selectedfrom the group consisting of a carbonate-based solvent, an ester-basedsolvent, an ether-based solvent, and a ketone-based solvent.

The lithium secondary battery, including the electrolyte solution,includes a cathode, an anode, and a separator interposed between thecathode and the anode, and the cathode of the lithium secondary batterymay include a nickel-cobalt-manganese (NCM)-based cathode activematerial, at this time, nickel, cobalt, and manganese may have a ratioof 6:2:2 to 8:1:1.

According to the present disclosure, when a lithium secondary battery ismanufactured using an electrolyte using additive 1 and additive 2,additive 2 firmly forms CEI and SEI in the cathode and anode, therebyobtaining a lithium secondary battery with increased electrochemicalcharacteristics.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing the energy levels of HOMO and LUMO of4-(allyloxy)phenyl fluoro sulfate;

FIG. 2 is a graph showing the initial cell efficiency for ComparativeExamples and Examples;

FIG. 3 is a graph showing the high-temperature life for ComparativeExamples and Examples; and

FIG. 4 is a graph showing high rate characteristics for ComparativeExamples and Examples.

DETAILED DESCRIPTION

Hereinafter, specific contents for solving the above-described objectiveand problems will be described in detail with reference to theaccompanying drawings. On the other hand, when the detailed descriptionof a known technology in the same field is not helpful in understandingthe core content of the disclosure in understanding the presentdisclosure, the description will be omitted, and the technical spirit ofthe present disclosure is not limited thereto and may be variouslyimplemented by being changed by those skilled in the art.

One feature of the present disclosure is that the electrochemicalproperties of a lithium secondary battery may be increased by usingvinylene carbonate (VC) represented by Formula 1 as an additive and4-(allyloxy)phenyl fluoro sulfate represented by Formula 2 as anadditive, simultaneously.

Formulae 1 and 2 are as follows.

Hereinafter, VC will be represented as additive 1, and4-(allyloxy)phenyl fluoro sulfate will be represented as additive 2.

By using additive 1 and additive 2 simultaneously, cathode electrolyteinterphase (CEI) is formed on the cathode, and SEI is formed on theanode, thereby improving the lifespan and output characteristics of thebattery.

Specifically, since additive 2 has a low LUMO energy level and a highHOMO energy level compared to other materials included in theelectrolyte, it is expected to react first on the cathode and anodesurfaces to form CEI and SEI.

FIG. 1 shows the energy levels corresponding to LUMO and HOMO ofadditive 2, and MH-161 corresponds to additive 2 of the presentdisclosure. Referring to FIG. 1 , the HOMO energy level of additive 2 is-5.96 eV, and the energy level of LUMO is -1.73 eV, and it may beexpected that the reaction will occur as it is easily decomposed at thecathode and the anode than EC and FEC used as solvents, VC used asadditives, and LiPO₂F₂.

In particular, the formation of a film can be expected by inducingradical polymerization through the vinyl group at the end of theadditive 2. The polymer formed as described above is a polymer componentand may have physical flexibility, thereby suppressing breakage of afilm structure due to volume expansion and contraction caused by ananode problem, and a phenomenon in which the film is continuouslythickened due to breakage of the film structure and the resultingexposure of the cathode active material.

In addition, the molecules are dissociated in the electrolyte to haveOSO²⁻ functional groups and are expected to form SEI of Li₂SO₃ andROSO₂LI components. The sulfone-based components such as Li₂SO₃ andROSO₂Li are expected to play a role in improving electrochemicalproperties by forming a film having low resistance and excellent thermalstability.

In addition, fluorine (F) bonded to the molecular end is expected toform LiF on the surface of the anode, and additive 2 is expected to bemore effective in forming LiF because fluorine has a higherdeintercalation tendency than other additives, fluoroethylene carbonate(FEC),(The binding energy of the C—F bond inside the FEC is -1.61 eV,and the binding energy of the S—F bond inside the additive 2 is -4.35eV).

On the other hand, additive 2 of the present disclosure may be preparedthrough the following mechanism:

① 4.50 ml, 29.96 mmol of 4-(allyloxy)phenol and 4.88 g, 14.98 mmol ofcesium carbonate are dissolved in 45 ml of tetrahydrofuran, and 8.91 g,44.95 mmol of 1.1′-sulfonyl diimidazole is added.

② After the mixture was stirred at room temperature for 12 hours,4-(allyloxy)phenyl 1H-imidazole-1-sulfonate(MH-159) in the form of atransparent oil was obtained through column chromatography (ethylacetate/hexanes: 3/7)(93% yield, 7.78 g).

As a result of H-NMR, it was confirmed that MH-159 could be obtained byobtaining the following result values:

¹H NMR (300 MHz, CDCl₃) δ 7.71 (t, J = 1.0 Hz, 1H), 7.28 (t, J = 1.5 Hz,1H), 7.16 (dd, J = 1.6, 0.8 Hz, 1H), 6.86-6.79 (m, 4H), 6.01 (ddt, J =17.3, 10.5, 5.3 Hz, 1H), 5.39 (dq, J = 17.3, 1.6 Hz, 1H), 5.30 (dq, J =10.5, 1.4 Hz, 1H), 4.50 (dt, J = 5.3, 1.5 Hz, 2H). ¹³C{¹H} NMR (101 MHz,CDCl₃) δ 158.4, 142.5, 137.7, 132.6, 131.4, 122.4, 118.5, 118.3, 116.0,69.3.

③ 7.78 g, 27.76 mmol of MH-159 was dissolved in 100 ml of acetonitrile,6.34 g, 49.97 mmol of silver(I) fluoride was added, and the mixture wasstirred at 80° C. for 12 hours, and then 4-(allyloxy)phenyl fluorosulfate(MH-161) in the form of yellow oil can be obtained through columnchromatography (ethyl acetate/hexanes: 3/7) (90% yield, 5.80 g).

As a result of H-NMR, it was confirmed that MH-161 could be obtained byobtaining the following result values.

¹H NMR (400 MHz, CDCl₃) δ 7.29 - 7.21 (m, 3H),▫7.01 - 6.91 (m, 2H), 6.04(ddt, J = 17.3, 10.5, 5.3 Hz, 1H), 5.42 (dq, J = 17.3, 1.6 Hz, 1H), 5.32(dq, J = 10.5, 1.4 Hz, 1H), 4.55 (dt, J = 5.3, 1.5 Hz, 2H). ¹⁹F{¹H} NMR(377 MHz, CDCl₃) δ 36.4.

Hereinafter, the results of experiments on electrochemical properties bymanufacturing a lithium secondary battery using the additive will bedescribed.

The cathode includes an NCM-based cathode active material made of Ni,Co, and Mn, and in particular, NCM811 was used in this embodiment. Asthe cathode active material may be used of LiCoO₂, LiMnO₂, LiNiO₂,LiNi_(1-x)Co_(x)O₂, LiNi_(0.5)Mn_(0.5)O₂, LiMn_(2-x)M_(x)O₄ (M is Al, Lior a transition metal), LiFePO, and the like, and all other cathodeactive materials that can be used for lithium secondary batteries may beused.

The cathode may further include a conductive material and a binder.

The conductive material is used to impart conductivity to an electrode.Any electronically conductive material without causing chemical changesin a configured battery can be used as a conductive material. Forexample, natural graphite, artificial graphite, carbon black, acetyleneblack, Ketjen black, carbon fiber, metal powder such as copper, nickel,aluminum, silver, metal fiber, and the like can be used as a conductivematerial, and one type or a mixture of one or more types of conductivematerials including polyphenylene derivatives may be used.

The binder serves to adhere the particles of the active material well toeach other or to the current collector, which is to mechanicallystabilize the electrode. That is, the active material is stably fixed inthe process of repeated intercalation and deintercalation of lithiumions to prevent the loosening of the bond between the active materialand the conductive material. The binder may include polyvinyl alcohol,carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose,polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride,a polymer including ethylene oxide, polyvinylpyrrolidone, polyurethane,polytetrafluoro ethylene, polyvinylidene fluoride, polyethylene,polypropylene, styrene-butadiene rubber, acrylated styrene-butadienerubber, epoxy resin, nylon, and the like, but is not limited thereto.

The anode includes any one or more of carbon (C)-based or silicon(Si)-based anode active material, and the carbon-based anode materialmay include one or more of materials selected from the group consistingof artificial graphite, natural graphite, graphitized carbon fiber,graphitized meso-carbon microbeads, fullerene, and amorphous carbon, andthe silicon-based anode active material may include any one of SiO_(x)and silicon-carbon composite materials. In particular, a graphite anodeactive material was used in this embodiment.

Like the cathode, the anode may further include a binder and aconductive material.

The electrolyte solution is composed of an organic solvent andadditives.

The organic solvent may include any one or two or more solvents selectedfrom the group consisting of a carbonate-based solvent, an ester-basedsolvent, an ether-based solvent, and a ketone-based solvent.

At this time, the carbonate-based solvent may include dimethyl carbonate(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propylcarbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate(EMC), ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), fluoroethylene carbonate (FEC), vinylene carbonate (VC),and the like. In addition, γ-butyrolactone (GBL), n-methyl acetate,n-ethyl acetate, n-propyl acetate, and the like may be used as theester-based solvent, and dibutyl ether may be used as the ether-basedsolvent but is not limited thereto.

The solvent may further include an aromatic hydrocarbon-based organicsolvent. Specific examples of the aromatic hydrocarbon-based organicsolvent may include alone or in combination of benzene, fluorobenzene,bromobenzene, chlorobenzene, cyclohexylbenzene, isopropyl benzene,n-butylbenzene, octyl benzene, toluene, xylene, mesitylene, and thelike.

The separator prevents a short circuit between the cathode and anode andprovides a passage for lithium ions to move. Such separators may includeknown materials such as polyolefin-based polymer membrane such aspolypropylene, polyethylene, polyethylene/polypropylene,polyethylene/polypropylene/polyethylene, andpolypropylene/polyethylene/polypropylene, or multilayers thereof,microporous films, woven fabrics, and non-woven fabrics. In addition, afilm coated with a resin having excellent stability on the porouspolyolefin film may be used.

Preparation and experiment of batteries corresponding to ComparativeExamples and Examples

Preparation of Cathode

For the preparation of the cathode, PVdF was dissolved in NMP to preparea binder solution.

A slurry was prepared by mixing the cathode active material, and KetjenBlack used as a conductive material in a binder solution. The slurry wascoated on both sides of an aluminum foil and dried.

After that, a rolling process and a drying process were performed, andthe aluminum electrode was ultrasonically welded to prepare a cathode.In the rolling process, the thickness was adjusted to be 120 µm to 150µm.

In this case, Li[Ni_(1-x-y)Co_(x)Mn_(y)]O₂ (1-x-y≥0.6), a material inwhich Ni, Co, and Mn were mixed in the ratio of an 8:1:1, was used asthe cathode active material.

Preparation of Anode

A slurry was prepared by mixing the anode active material, and KetjenBlack used as a conductive material in a binder solution. The slurry wascoated on both sides of an aluminum foil and dried.

After that, a rolling process and a drying process were performed, andthe aluminum electrode was ultrasonically welded to prepare a cathode.In the rolling process, the thickness was adjusted to be 120 µm to 150µm.

At this time, graphite was used as the anode active material.

Preparation of Electrolyte Solution

A mixture of ethylene carbonate (EC), ethylmethyl carbonate (EMC), anddiethyl carbonate (DEC) in a volume ratio of 25:45:30 was used as anorganic solvent, and 0.5 M LiPF₆ and 0.5 M LiFSI were dissolved in thesolvent as lithium salts, and the electrolyte was injected. In addition,according to each Example, different ratios of additive 2 were added tothe organic solvent.

Preparation of Coin Cell

After interposing a separator between the cathode and the anode, andthen wound to prepare a jelly roll. A coin cell was prepared using theprepared jelly roll and electrolyte.

Comparative Example 1

Only additive 1 (1.0% by weight) was used as an additive in theelectrolyte, and it is a lithium secondary battery that does not includeadditive 2.

Comparative Example 2

It is a lithium secondary battery using an electrolyte solution furtherincluding additive 1 (1.0% by weight) and LiPO₂F₂ (0.5% by weight) as anadditive.

Example 1

It is a lithium secondary battery using an electrolyte solutionincluding additive 1 (1.0% by weight) and additive 2 (0.1% by weight).

Example 2

It is a lithium secondary battery using an electrolyte solution furtherincluding additive 1 (1.0% by weight) and additive 2 (0.5% by weight).

Example 3

It is a lithium secondary battery using an electrolyte solutionincluding additive 1 (1.0% by weight) and additive 2 (0.1% by weight).

For Comparative Examples 1 to 2 and Examples 1 to 3, the results ofmeasuring the cell initial charging and discharging efficiency, thelifespan characteristics after 100 cycles of charging and discharging ata high temperature (45° C.), and the rate-specific characteristics at ahigh rate (2 C-rate) are shown in Tables 1, 2 and 3.

TABLE 1 Additives (% by weight) Cell initial efficiency (%) Additive 1LiPO₂F₂ Additive 2 Comparative Example 1 1.0. - - 96.4% ComparativeExample 2 1.0. 0.5. - 96.5% Example 1 1.0. - 0.1. 92.5% Example 2 1.0. -0.5. 97.1% Example 3 1.0. - 1.0. 92.5%

Table 1 shows the cell initial efficiencies for Comparative Examples andExamples, and the cell initial efficiency refers to a value obtained bydividing a discharge capacity by a charge capacity after charging onceafter the manufacturing of a lithium secondary battery is completed,discharging is performed, and then discharging is performed. The cut-offvoltage was set to 2.5 V to 4.2 V, and the C-rate was tested at 1C at45° C.

As a result of the experiment, in the case of Example 2 in which 0.1% byweight of additive 2 was added, it was confirmed that the initial cellefficiency was the best. In the case of Examples 1 and 3, experimentalresults were found to be inferior to those of the comparative example.

A graph for this is shown in FIG. 2 .

TABLE 2 Additives (% by weight) High-temperature lifespan @100cycleAdditive 1 LiPO₂F₂ Additive 2 Comparative Example 1 1.0. - - 90.8%Comparative Example 2 1.0. 0.5. - 90.9% Example 1 1.0. - 0.1. 92.0%Example 2 1.0. - 0.5. 92.4% Example 3 1.0. - 1.0. 90.9%

Table 2 shows a high-temperature life for Comparative Examples andExamples and shows how much charging/discharging capacity may bemaintained compared to an initial charging/discharging capacity after100 cycles of charging and discharging are repeated. The cut-off voltagewas set to 2.5 V to 4.2 V, and the C-rate was tested at 1 C at 45° C.

As a result of the experiment, in the case of Example 2 in which 0.1% byweight of additive 2 was added, it was confirmed that the initial cellefficiency was the best. Example 1 showed a high-temperature lifespansimilar to that of Comparative Example 2, and Example 3 showed ahigh-temperature lifespan lower than that of Comparative Examples.

A graph for this is shown in FIG. 3 .

TABLE 3 Additives (% by weight) Rate-specific characteristics @2C-rateAdditive 1 LiPO₂F₂ Additive 2 Comparative Example 1 1.0. - - 85.7%Comparative Example 2 1.0. 0.5. - 86.1% Example 1 1.0. - 0.1. 86.3%Example 2 1.0. - 0.5. 87.1% Example 3 1.0. - 1.0. 87.0%

Table 3 shows the rate-specific characteristics for Comparative Examplesand Examples and shows how much charge/discharge capacity can bemaintained compared to the existing 1C-rate by increasing the rate by 2times compared to other experiments. Likewise, the cut-off voltage wasset to 2.5 V to 4.2 V, and the experiment was performed at 45° C.

As a result of the experiment, in the case of Example 2 in which 0.1% byweight of additive 2 was added, it was confirmed that the initial cellefficiency was the best.

A graph for this is shown in FIG. 4 .

Through the above experiment, when a lithium secondary battery ismanufactured using an electrolyte solution using both additive 1 andadditive 2, a lithium secondary battery with improved electrochemicalproperties can be obtained. This is presumed to be due to the fact thatadditive 2 strongly forms CEI and SEI at the cathode and anode.

From the experimental results, it was confirmed that additive 1 andadditive 2 showed the highest electrochemical properties when added in aratio of 2:1.

Although shown and described with respect to specific embodiments of thepresent disclosure, it is within the art that the present disclosure canbe variously improved and changed without departing from the spirit ofthe present disclosure provided by the following claims. It will beobvious to those of ordinary skilled in the art.

1. An electrolyte solution for a lithium secondary battery, theelectrolyte solution comprising: an electrolyte salt; and an organicsolvent; wherein the electrolyte solution further comprises, asadditives, vinylene carbonate (VC) represented by Formula 1:

and 4-(allyloxy)phenyl fluoro sulfate represented by Formula 2:

.
 2. The electrolyte solution of claim 1, wherein the 4-(allyloxy)phenylfluoro sulfate comprises 0.1 to 1.0% by weight with respect to the totalweight of the electrolyte solution.
 3. The electrolyte solution of claim1, wherein the VC comprises 0.1 to 10% by weight with respect to thetotal weight of the electrolyte solution.
 4. The electrolyte solution ofclaim 1, wherein the 4-(allyloxy)phenyl fluoro sulfate and the VC areadded in a ratio of 1:2.
 5. The electrolyte solution of claim 1, whereinthe electrolyte salt is any one compound or a mixture of two or morecompounds selected from the group consisting of: LiPF₆, LiBF₄, LiClO₄,LiCl, LiBr, LiI, LiB₁₀Cl₁₀, LiCF₃SO₃, LiCF_(3.0)CO₂, Li(CF₃SO₂)_(3.0)C,LiAsF₆, LiSbF₆, LiAlCl₄, LiCH₃SO₃, LiCF₃SO₃, LiN(SO_(2.0)C₂F₅)₂,Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiB(C₆H₅)₄, and Li(SO₂F)₂N(LiFSI).
 6. Theelectrolyte solution of claim 1, wherein the electrolyte salt iscomprised in a concentration range of 0.5 M to 1.0 M.
 7. The electrolytesolution of claim 1, wherein the organic solvent is any one or a mixtureof two or more solvents selected from the group consisting of: acarbonate-based solvent, an ester-based solvent, an ether-based solvent,and a ketone-based solvent.
 8. A lithium secondary battery comprising acathode, an anode, a separator interposed between the cathode and theanode, and the electrolyte solution of claim
 1. 9. The lithium secondarybattery of claim 8, wherein the cathode comprises anickel-cobalt-manganese-based cathode active material.
 10. The lithiumsecondary battery of claim 9, wherein in the cathode active material,nickel, cobalt, and manganese are contained in a ratio of 6:2:2 to8:1:1.